This invention relates to ceramic structures which have high resistance to thermal shock.
Many electric utility companies maintain standby gas-fired generators which are reserved for use in times of peak demand or emergency shutdown of other generating facilities. Such generators employ large compressors to force air through combustion zones where natural gas is burned. The resulting heated combustion product is used to drive turbines for the generation of electricity.
The temperature of a natural gas flame at its hottest point is sufficient to form some nitrogen oxides which are objectionable from an environmental point of view. Treatment of the entire exhaust gas stream to remove nitrogen oxides would be prohibitively expensive. One alternative would be to accomplish combustion of the natural gas with the use of a catalyst, which could decrease the combustion temperature to a level at which nitrogen oxides are not created. However, the support structure for the catalyst must be able to withstand the temperature of catalyzed gas combustion, about 1260.degree. C., and must also be able to withstand repeated cycles of thermal shock created when the generator is started up and is shut down. When such a generator is shut down, the gas supply is cut off, but compressed air continues to flow through the combustion chamber, cooling the catalyst support very rapidly. A catalyst support structure in such a situation might be subjected to a typical temperature change of from 1260.degree. to 300.degree. C. in only 0.02 to 0.1 seconds.
In addition to the requirement of withstanding severe thermal shock, the catalyst support structures must not fail in a manner that would damage the downstream turbine blades. Thus the desirable ceramic structure must not only be able to withstand the very severe temperature fluctuations without breaking, but if it does break, it must fracture into small highly frangible and therefore harmless particles.
The use of ceramic honeycomb structures as catalyst supports is well known in the art. U.S. Pat. No. 4,092,194 and U.S. Pat. No. 3,986,528 describe tubes of multiple layers of ceramic fibers bonded to one another at the points where the fibers cross. The materials used to make these tubes can include alumina or alumina precursors and S glass, so reaction products of these materials may also be present. These structures have channels which are not discrete, but which interconnect between the crossing points of the yarn. See also U.S. Pat. No. 3,949,109 which discloses similar structures of partly sintered glass-ceramic fibers.
Other references disclose extruded ceramic honeycomb shapes. See for example U.S. Pat. No. 4,869,944 which describes a ceramic structure consisting of SiO.sub.2, Al.sub.2 O.sub.3 and MgO, primarily in the form of cordierite. The structure has microcracks which is said to help absorb thermal expansion and thus contribute to resistance to thermal shock. Other extruded or molded structures are taught in U.S. Pat. No. 4,069,157. This patent lists alumina, mullite and cordierite as being useful alternative materials for the manufacture of the structures, but states that the structures can be used at the relatively low temperature of 300.degree. C. (Col. 2, line 3). U.S. Pat. No. 3,255,027 teaches refractory structures which may include honeycombs (col. 4, line 61) made from alumina, and which may include other components such as silica and mullite. Other commonly assigned patents with disclosures of alumina refractories possibly including components such as silica and mullite are U.S. Pat. Nos. 3,311,488, 3,298,842 and 3,244,540.
None of the references teach ceramic structures capable of surviving the severe conditions described above. Further, the monolithic molded or extruded structures of these references could pose risk of damage to turbine blades in the event they failed, fracturing into large pieces.